Web multiple panels consisting of two parallel planar outside walls (and, optionally, inner walls parallel to them) and integral webs placed between the outside walls have achieved considerable technical importance as light, heat-insulating, rigid glazing or roofing material. Because of the stiffening action of the webs, the web multiple panels can be laid freely over great support spans in the web direction. However, the stiffness crosswise to the web direction leaves something to be desired. Web multiple panels with undulated webs in the extrusion direction do have a higher crosswise stiffness. However, because of the great equipment costs for their production, they have not achieved any technical importance so far.
A process for producing hollow profile panels with undulated intermediate webs by use of a special extrusion die is known from Swiss Pat. No. 457,829. The undulated arrangement is achieved by rhythmic lateral movement of the cores between which the webs are formed within the die orifice. While the intermediate webs within the panel area are thus extruded in an undulated form, the side web is produced as a full profile of varying width, which causes a considerable consumption of material and undesirable stresses in the material. Also, the rhythmic movement of the die core provided with the core piece is technically costly.
Another principle for undulation of the webs is shown in German Patent Document DE-OS No. 27 34 464. Here also a specially designed extrusion die is necessary. By a rhythmic variation of the flow resistance of the molding material in the die orifice during extrusion of the hollow extrudate, the outside wall compartments of the web multiple panel which are located between the attachment sites of the webs are alternately made thicker and thinner so that in each case a thick and thin zone are next to each other in neighboring compartments. Then the extrudate in thermoplastic state is stretched biaxially or triaxially, whereby the thick zones are stretched less than the thin zones. Thus, during stretching the web moves to different distances from one another, which leads to an undulation in the rhythm of the thick and thin zones that follow one another. In this case, the webs that are next to one another are always undulated in opposite direction.